This invention relates to the field of cathode ray tube circuitry and, more particularly, to means for preventing damage to the output transistor in the horizontal scanning circuit of a CRT device, or to the CRT screen.
In the field of CRT devices it is well known that the horizontal output transistor which supplies the horizontal deflection current to the yoke is subject to damage under a number of circuit conditions. For example, if a disrupted drive signal were to be applied to the driver transistor in the circuit, the output transistor would be caused to conduct during a portion of the flyback time. In that event, the collector current of the output transistor could exceed safe limits. Naturally, if the CRT device uses an internal oscillator which is merely synced by the incoming horizontal sync pulses, this problem would rarely, if ever, occur. Such devices depend on the free-running frequency of the oscillator for such protection. However, in directly driven systems, where each horizontal deflection is dependent solely on an incoming pulse, the situation is somewhat different. This is typically the situation in data display monitors where the "direct drive" signals come from the logic circuitry which develops the timing sequence which is necessary in order to generate the characters of the data display. During initialization or due to a received "spike" or to an interruption in the supply voltage, the logic circuitry may "miscount" or change frequency. Also, in a direct drive horizontal deflection system, if the B+ and drive signal should be terminated at the same time, the total residual charge could damage the CRT screen (known as "spot burn"). The flyback voltage is usually rectified to supply a high DC voltage to the CRT screen. If this voltage is allowed to remain on the CRT when B+ and the drive signal are removed, a high intensity electron beam will be produced, aimed at the center of the screen. This problem is often prevented by including a bleeder resistor to discharge the high DC voltage before the screen can be damaged. This solution has two disadvantages; namely, the relatively high cost of such a resistor, and the load added to the circuit by the resistor.
Another known solution, known as a "spot kill" circuit, uses the turn-off of B+ and drive to turn on a transistor which will put a bias on the grid of the CRT. This grid voltage, applied at the same time that the above-mentioned high intensity beam is produced, causes a harmless "flash" over the entire screen instead of a burn at the center of the screen. The flash discharges the stored high voltage. The simplest known "spot kill" circuit includes at least one transistor and a diode, plus resistors and capacitors and is, therefore, a relatively expensive solution also.